The woman's weight must be on the ball of the foot so that she can pivot as if on a fixed pin. But she does not pivot her whole body. She only pivots her lower body from the waist down. The waist is like the swivel that joins the upper body and the lower body. Since her upper body must face and connect to the man in front of her, she has to turn her lower body sideways in order to dance around him. This is known as “dissociation”.
The rolling of the chest is caused by the rotation of the hips. To create the gear effect, the woman has to swivel her hips fully until her chest rolls along. She needs to make the rolling void of abruptness and bumpiness so it feels smooth, musical and comfortable, which is not easy to do and needs a lot of practice to master. A beginner who can't do dissociation often keeps her hips still and crosses her leg instead. Consequently, her chest sticks on his torso and does not trundle. Tango is a dance in which both partners pleasure each other with their bodies. An experienced woman knows how to use her body to seduce the man, just like an experienced man knows how to display the feminine beauty of her body. (See Revealing her Beauty in Tango.) Gear effect increases the sensual pleasure of the dance - a feature of close-embrace tango that is missing in the open-embrace style. It is one of the things that make the two styles fundamentally different.